Herpesviruses are etiologic agents or co-factors for certain cancers. In addition, reactivations of herpesviruses from the latent state are frequent in cancer patients and can result in severe and debilitating diseases. The long-range goals of this project are to understand the mechanisms by which herpes simplex virus (HSV) induces cell fusion, a process which facilitates the spread of virus in the human body as well as in cell culture. Experiments have been designed to define the viral and cell factors that influence whether cell fusion will occur after infection. The specific aims are (i) to determine which HSV proteins are necessary and sufficient to induce cell fusion; (ii) to identify viral and cell proteins that interact with cytoplasmic domains of two membrane glycoproteins (gB and gH) known to be required for cell fusion; (iii) to dissect the structure and function of HSV proteins, such as UL45, that may be required for cell fusion but not for infectivity; (iv) to identify the viral protein that interacts with glycosaminoglycans resulting in the activation of cell fusion; and (v) to investigate the role in cell fusion of a newly identified cell surface receptor that enhances HSV-1 entry into cells. The experimental approaches will include construction of plasmids expressing wild-type and mutant forms of viral proteins, for use in cell fusion assays; use of the two-hybrid method and co-immunoprecipitation to investigate protein-protein interactions; isolation and characterization of viral mutants and use of Chinese hamster ovary cells expressing a human cell surface receptor in cell fusion assays.